Conceivably, we could serve the normal site to search engine spiders and black it for everyone else. However, we have a complex load balancing setup that may make this impossible.
An all-black theme would do the trick, but redirecting using Temporary Redirect shouldn't screw up search if we do it right.
Your best option would be a 302 redirect to a SOPA-specific page. Google won't follow that and it shouldn't screw your search engine rankings. At least, that's how I understand it.
I wouldn't really want to count on that; Google tends to guess what common response codes mean. The safest response would probably be to use "503 Service Unavailable" with an error page.
An all-black theme would do the trick, but redirecting using Temporary Redirect shouldn't screw up search if we do it right.